Herpesvirus infections are common and sometimes serious after human marrow transplantation. Prospective studies of the lymphocyte transformation response to HSV, VZV and CMV antigens showed that they are depressed immediately after transplant and do not recover until active virus infection occurs. For CMV in particular, recovery of this response did not seem to determine recovery from virus infection. Prospective studies of cell-mediated cytotoxicity to CMV-infected and uninfected cells and to K562 cells are proposed to determine the importance of this response (or responses) in antiviral immunity after marrow transplant. A randomized study of an hyperimmune globulin for the prevention of CMV infection continues. Early results have shown that globulin is ineffective in patients receiving white cell transfusions from CMV seropositive donors, but there may be an effect in patients not receiving white cell transfusions. Finally, treatment of CMV interstitial pneumonia with human leukocyte interferon alone and in combination with adenine arabinoside was unsuccessful. However, the toxicity data from these studies has been used in the initiation of a prophylactic trial of leukocyte interferon after marrow transplant.